Remove recipe for RocksDB and add recipe for Snappy (#4431)

- Copies the recipe for Snappy from Conan Center, but removes three
  lines that explicitly link the standard library, which prevents
  builders from statically linking it.
- Removes the recipe for RocksDB now that an official recipe for version
  6.27.3 is in Conan Center.

Developers will likely need to remove cached versions of both RocksDB
and Snappy:

```
conan remove -f rocksdb
conan remove -f snappy
```

---------

Co-authored-by: John Freeman <jfreeman08@gmail.com>

.github/workflows/nix.yml

@@ -69,8 +69,8 @@ jobs:
         with:
             path: ${{ steps.conan-cache.outputs.dir }}
             key: ${{ hashFiles('~/.conan/profiles/default', 'conanfile.py', 'external/rocksdb/*', '.github/workflows/nix.yml') }}
-      - name: export RocksDB
-        run: conan export external/rocksdb
+      - name: export Snappy
+        run: conan export external/snappy snappy/1.1.9@
       - name: install dependencies
         run: |
           mkdir ${build_dir}

.github/workflows/windows.yml

@@ -69,8 +69,8 @@ jobs:
         with:
             path: ${{ steps.conan-cache.outputs.dir }}
             key: ${{ hashFiles('~/.conan/profiles/default', 'conanfile.py', 'external/rocksdb/*', '.github/workflows/windows.yml') }}
-      - name: export RocksDB
-        run: conan export external/rocksdb
+      - name: export Snappy
+        run: conan export external/snappy snappy/1.1.9@
       - name: install dependencies
         run: |
           mkdir $env:build_dir